This paper presents an explanation of why methyl tert-butyl ether (MTBE) production by reactive distillation may yield multiple solutions. Widely different composition profiles and conversions may, as already-reported by Krishna and others, result with identical column specifications, depending on the initial estimates provided. A hypothesis yielding a qualitative understanding of this phenomenon has been developed, The inert n-butene plays a key role in the proposed explanation : As the reaction mixture is diluted with n-butene, the activity coefficient of methanol increases substantially and the temperature decreases. This dilution has a profound effect on the equilibrium conversion, enabling MTBE to escape from the reactive zone without decomposition. When methanol is fed below or in the lower part of the reactive zone of the column, the "lifting capacity" of the minimum boiling point MTBE-methanol azeotrope will also be important.